Micro assembler with fine angle control

ABSTRACT

An assembly surface has an array of electrodes arranged such that each of a plurality of chiplets can be positioned to cover at least one of the electrodes. A field generator applies a rotation field that is orthogonal to the clamping force field applied by the electrodes. A processor is operable to determine a desired orientation angle of a first subset of the chiplets and activate one or more of the electrodes so that a second subset of the chiplets different than the first subset is kept from rotating by a clamping force field applied by the one or more of the electrodes. While the clamping force field is being applied, the processor applies the rotation field at the selected angle to cause the first subset of the chiplets to be oriented at the desired orientation angle.

SUMMARY

The present disclosure is directed to a micro assembler with fine anglecontrol. In one embodiment, an apparatus includes an assembly surfacewith an array of electrodes arranged such that each of a plurality ofchiplets can be positioned to cover at least one of the electrodes. Eachof the electrodes is capable of being individually activated to apply aclamping force field to one of the chiplets. A field generator applies arotation field that is orthogonal to the clamping force field applied bythe electrodes. The field generator has an input that selects an angleof the rotation field. A processor is coupled to the electrodes and thefield generator. The processor is operable to determine a desiredorientation angle of a first subset of the chiplets and activate one ormore of the electrodes so that a second subset of the chiplets differentthan the first subset is kept from rotating by the clamping force fieldapplied by the one or more of the electrodes. While the clamping forcefield is being applied, the processor causes the rotation field to beapplied at the selected angle to cause the first subset of the chipletsto be oriented at the desired orientation angle.

In another embodiment, a method involves arranging a plurality ofchiplets on an assembly surface such that the chiplets do not interferewith one another. The assembly surface has an array of electrodes andthe chiplets arranged such that each chiplet is over at least one of theelectrodes. A desired orientation angle of a first subset of thechiplets is determined, and one or more of the electrodes is activatedso that a second subset of the chiplets different than the first subsetis kept from rotating by a clamping force field applied by the one ormore electrodes. A rotation field (e.g., magnetic field) that isorthogonal to the clamping force field is applied by the one or moreelectrodes. An angle of the rotation field is selectable to cause thefirst subset of the chiplets to be oriented at the desired orientationangle.

These and other features and aspects of various embodiments may beunderstood in view of the following detailed discussion and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The discussion below makes reference to the following figures, whereinthe same reference number may be used to identify the similar/samecomponent in multiple figures. The drawings are not necessarily toscale.

FIG. 1 is a diagram of a micro assembler apparatus according to anexample embodiment;

FIG. 2 is an isometric view of a micro assembler according to anotherexample embodiment;

FIG. 3 is a close up view of the micro assembler shown in FIG. 2;

FIGS. 4 and 5 are diagrams showing the adjustment of field angle bychanging coil currents according to an example embodiment;

FIG. 6 is an isometric view of a micro assembler according to anotherexample embodiment;

FIGS. 7, 8, and 9 are signal diagram showing operation of a microassembler according to example embodiments and

FIG. 10 is a flowchart of a method according to an example embodiment.

DETAILED DESCRIPTION

The present disclosure relates to automated assembly of micro objectswith fine orientation control. One particular example is assembly ofthin materials. Certain classes of materials, such as van der Waalsmaterials or two-dimensional (2-D) materials may formed as a sheet thatcan be as thin as one molecule in thickness. One well-known example of a2-D material is graphene, which is a single-molecule lattice of carbonatoms. Many 2-D materials have uncommon properties (e.g., mechanicalstrength, low thermal and electrical resistance, etc.) and so there isinterest in the development of devices that use 2-D materials.

In order to mass-produce devices using 2-D materials, automated systemsare needed to, among other things, manufacture bulk 2-D material (e.g.,via exfoliation or chemical layer deposition), divide the bulk materialinto individual units of the desired size, and move the units into adesired location and orientation, and then manipulate the units to formand assembly (e.g., stack 2-D materials together and/or assemble withother components). Methods and apparatuses have been devised to separatethe materials into individual units (referred to herein as ‘chiplets’)and move the chiplets into a desired location, e.g., a grid that allowsthe chiplets to be picked up individually or collectively. Note that thepresent embodiments are not limited to the assembly of 2-D materials,and may be applicable to any mass assembly process that involvesautomatically orienting large numbers of small objects such as chiplets.

The present disclosure relates to methods and apparatuses used to moveand rotate chiplets into a desired orientation. In FIG. 1, an isometricview shows an apparatus 100 according to an example embodiment. Theapparatus 100, also referred to herein as a micro assembler used tomanipulate chiplets 102. The chiplets are moved onto an assembly surface104 that includes an array of electrodes 106. A layer of dielectricfluid may also be spread across the assembly surface to reduce friction.

A processor 108 selectively sends voltages to the electrodes 106 toarrange the chiplets 102 to the specific locations on the assemblysurface 104. Electrostatic force generated by the electrode array tomove chiplets to the specific locations on the assembly surface 104along first and second directions 110, 111 (e.g., arbitrarily defined xand y directions). Since the chiplets 102 usually provide significantconductivity difference to a dielectric fluid medium that is on thesurface 104, induced dipoles align the long side of the chiplet to thenet electric field direction. However, since the electrode arraypixelates electric fields in the 2D spatial plane (generally along theassembly surface 104), it has been found that it will be difficult toachieve continuous fine angle orientation using this structure.

The illustrated apparatus can achieve very fine arbitrary angle controlfor individual chiplets 102 via a rotational field 114 that isorthogonal to electrostatic forces that are used for assembling chiplets102 to the desired X-Y positions on the assembly surface. Block 118represents a field generator that applies the rotation field 114 that isorthogonal to a clamping force field 116 applied by the electrodes 106.The field generator includes an input 122 (e.g., a signal line) thatselects an angle of the rotation field 114. The processor 108 is coupledto the field generator 118 and operable to determine a desiredorientation angle of a first subset of the chiplets 104. The processor108 is coupled to the field generator 118 and electrodes 106 (and otherdevices described herein, such as sensors) via interface circuitry thatmay include any combination of input/output busses, preamplifiers, poweramplifiers, digital to analog converters, analog to digital converters,filters, digital signal processors, etc.

One or more of the electrodes are activated so that a second subset ofthe chiplets different than the first subset is kept from rotating bythe clamping force field 116 applied by the one or more of theelectrodes 106. While the clamping force field 116 is being selectivelyapplied to the second subset of chiplets, the rotation field is appliedat the selected angle to cause the first subset of the chiplets to beoriented at the desired orientation angle.

Note that the spacings of the electrodes 106 are defined relative to thedimensions of the chiplets 102 so that a chiplet 102 overlaps at leastone electrode 106. The processor 108 (e.g., using optical image captureto detect xy-location and angle of each chiplet 102 relative to surface104) can determine which chiplets 102 overlay which electrodes 106. Thiscan be used to form a mapping of chiplets 104 to electrodes 106. Thismapping can be used to selectively energize electrostatic clamping 116forces on subsets of the chiplets 102, while others not in the subsetscan be rotated via the field generator 118.

In order for the chiplets 102 to be affected by the rotational field114, they may be formed so as to predictably respond to the field 114.For example, if the rotational field 114 is a magnetic field, then thechiplets 102 may be mated to magnetic materials such as ferromagneticstrips. The chiplets 102 themselves may instead or in addition have ananisotropic magnetic response. Other rotation fields, such as anacoustic field, may not require any special adaptation or inherentproperties of the chiplets 102.

In FIG. 2, an isometric view shows a micro assembler 210 that utilizesmagnetic force according to an example embodiment. Magnetic forces canbe readily generated and controlled by electrical coils. In this examplea first pair 200, 201 and a second pair 202, 203 of Helmholtz coils areoriented 90 degrees to each other. The first pair 200, 201 of coils arelocated on a first pair of opposite sides 204 a, 204 b of an assemblysurface 204 and the second pair of coils 202, 203 located on a secondpair of opposite sides 204 c, 204 d of the assembly surface.

The coils 200-203 generate uniform B field with arbitrary angles basedon the current ratio of the first and second currents sent to therespective first pair 200, 201 and second pair 202, 203. The arbitraryangle, uniform B field will be applied to all the chiplets 300 on theassembly surface 204, as shown in the detail view in FIG. 3. Thesechiplets may have strips of patterned ferromagnetic material, such asNi, embedded such that chiplets 300 will align to the B-field directionwhen they are not clamped down by the attraction force generated byassembler electrode array. For example, all the chiplets 300 can berotated in parallel until the desired angles have been reached. When thedesired angles are reached for a subset of the chiplets, the assemblerelectrodes 302 underneath the subset are activated to produce a strongdielectrophoretic (DEP) attraction force. The attraction force clampsthe subset of chiplets 300 to the surface 204 and prevents them fromrotating any further. The B field can keep rotating until all thedesired angles of the chiplets 300 have been reached. Note that it ispossible to partially clamp chiplets to be rotated so that a rotationpoint (e.g., a centroid) of a chiplet is fixed while the chiplet isrotated.

The actuation electrodes 302 to generate the clamping DEP force will bechosen such that they are fully inside the chiplet-covering area so asto prevent any induced electric field interfering the chiplet edge andchiplet angle. Since the current ratio of the Helmholtz pairs 200-203can be controlled precisely with precision digital-to-analog converters(DAC) and quality electronics, it is possible for this arrangement toprovide angle control of chiplets on the order of 0.1 degrees.

In FIGS. 4 and 5, diagrams illustrate an example of how magnetic fieldscan be changed in the micro assembler 210 shown in FIGS. 2 and 3. Powersources 400-403 are shown as meters that schematically representmagnitude and direction of electrical current applied to respectivecoils 200-203. Note that four individual sources 400-403 are notstrictly needed, e.g., a single current source can be used for each pair200-201, 202-203, e.g., by wiring the coils within the pairs in seriesor in parallel with a power source.

As seen in FIG. 4, current applied to all four coils in differentdirections results in a first angle 406 between the resultant magneticfield 404 and the assembly surface 204. As seen in FIG. 5, a differentcombination of currents, in this case with current from sources 400 and401 being zero, results in a different angle 500, one that is alignedwith the field 504 generated by coil pair 202 and 203.

In FIG. 6, an isometric view shows a micro assembler 600 that utilizesmagnetic force according to another example embodiment. An assemblysurface 602 is between a pair 604, 605 of magnets, e.g., Helmoltz coils,permanent magnets. A structure 606 holds the magnets 604, 605 inposition relative to the surface 602, and, as indicated by arrow 607,one or both of the structure 606 and surface 602 can rotate, e.g., via amotor. This relative rotation between the magnets 604, 605 and thesurface allows selectively changing an angle 610 between the surface 602and a magnetic field 608 generated by the magnets 604, 605.

In FIG. 7, a signal diagram illustrates inputs to a micro assembleraccording to an example embodiment. Trace 700 represents a desired anglebetween the assembly surface and the rotation field, and in this exampleis an analog or digital value that ranges from −180 to 180, representinga 360 degree range of rotation. Trace 702 represents a magnitude of therotation field, ranging from zero to some maximum value. Note that for amicro assembler as shown in FIG. 2, these signals 700, 702 could betranslated, e.g., via a logic circuit, to first and second currentsapplied to first and second coil pairs. In such a configuration, thefirst and second currents could range from −A to +A, where A is amaximum current magnitude.

Trace 704 represents a voltage applied to selected set of electrodes toapply a clamping field. This trace 704 is simultaneously accompanied bya selection signal 705 that selects a subset of the electrodes, suchthat the current represented by trace 704 is only applied to thoseelectrodes. For purposes of this disclosure, a subset of the chiplets isintended to indicate at least one chiplet selected from all of thechiplets on the assembly surface, and fewer than all of the chiplets.For this example, the signal 705 represents a group of clampingelectrodes being activated. In another implementation, the signal 705could be represented by an n-bit number, each bit corresponding to oneof the n-electrodes. If a bit in the number is one, the electrode isselected and current from trace 704 is applied. Otherwise, if the bit iszero the electrode is deselected an no current is applied.

During time periods 706-709, the clamping force is applied to theelectrodes and the field activated at a particular rotation angle. Inthis simple example, the angle changes from around 180 degrees in timeperiod 706 to −180 degrees in time period 709. Note that during eachperiod 706-709, there is a delay (e.g., 710) between activating theclamping force via signal 704 and applying the rotation field via 702,which allows some time for the clamping forces to stabilize beforeapplying the rotation force. Note that this example is for purposes ofillustration and not limitation, and the selective rotation can beachieved using alternate signals and sequences. For example, the changein the angle signal 700 between time periods 706-709 need not be asteady increase or decrease, but may change in any order.

In FIG. 8, a signal diagram shows microassembler operations according toanother example embodiment. In this case, the signals 800, 802, 804, 805change continuously or discretely within a single time period 806.Rotation signal 800 differs in this example in that it represents anunsigned number from 0 to 180 degrees, and the field signal 802 canchange from positive to negative to enable the remaining rotationbetween 0 and −180 degrees. The clamping force current 804 remains onduring the entire period 806, while the selection signal 805successively activates more electrodes and after chiplets associatedwith the electrodes are aligned at desired angles.

In order to assist in smooth movement of the chiplets over the assemblysurface, the assembly surface may be covered by a thin layer ofdielectric fluid. Even with such a fluid, there may be small forces thattend to inhibit movement of the chiplets. These forces are referred toherein as ‘stiction,’ and may be caused by any combination of phenomenasuch as friction, stray electrostatic forces, fluid viscosity, surfacetension, etc. In FIG. 9, a signal diagram shows how a rotation fieldaccording to an example embodiment can be configured to help overcomestiction between the chiplets and the assembly surface. This exampleuses an angle signal 900, rotation field signal 902 and clamping fieldsignal 904 similar to the example in FIG. 7.

During times when the rotation field signal 902 is activating therotation field, modulations 900 a-d are applied to the angle signal 900,e.g., via processor and associated signal conditioning circuitry. Thesemodulations result in time-varying the direction of the magnetic fieldaround the target angle of the rotation field to induce small movementsin the first subset of the chiplets and assist in the chiplets reachingthe target angle. The small movements overcome stiction between thefirst subset of the chiplets and the assembly surface. Note that othersignals could be modulated similar to the angle signal 900. For example,the field signal 904 could be similarly modulated when it is activated.In other embodiments, the clamping field signal 904 could be quickly andselectively applied to and released from targeted chiplets to inducemovements that assist in the chiplets reaching the target angle.

The modulations 900 a-d applied to this or other signals could be anytype of waveform such as sinusoidal, sawtooth, square wave, random,impulse, exponentially decaying, etc. The modulations 900 a-d may haveone or more base frequencies that are selected to excite movement for aparticular type of field, chiplet, assembly surface, surface fluid, etc.These frequencies may be known beforehand, or be determined duringoperation, e.g., by sweeping through a range of frequencies andobserving (e.g., via optical recognition) magnitude of rotation for eachfrequency in the range.

In FIG. 10, a flowchart shows a method according to an exampleembodiment. The method involves arranging 1000 a plurality of chipletson an assembly surface, e.g., such that the chiplets do not interferewith one another. The assembly surface has an array of electrodes andthe chiplets are arranged such that each chiplet is over at least one ofthe electrodes. A desired orientation angle of a first subset of thechiplets is determined 1001. One or more of the electrodes is activated1002 so that a second subset of the chiplets (different than the firstsubset) is kept from rotating by a clamping force field applied by theone or more electrodes. A rotation field is applied 1003 that isorthogonal to the clamping force field applied by the one or moreelectrodes. An angle of the rotation field is selectable to cause thefirst subset of the chiplets to be oriented at the desired orientationangle. As indicated by line 1004, these processes of determining anorientation 1001, activating 1002 the electrodes, and applying 1003 therotation field can be repeated for increasingly smaller subsets of thechiplets, while adding to the number of chiplets that are being clampedby the electrodes.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein. The use of numerical ranges by endpointsincludes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2,2.75, 3, 3.80, 4, and 5) and any range within that range.

The foregoing description of the example embodiments has been presentedfor the purposes of illustration and description. It is not intended tobe exhaustive or to limit the embodiments to the precise form disclosed.Many modifications and variations are possible in light of the aboveteaching. Any or all features of the disclosed embodiments can beapplied individually or in any combination are not meant to be limiting,but purely illustrative. It is intended that the scope of the inventionbe limited not with this detailed description, but rather determined bythe claims appended hereto.

The invention claimed is:
 1. An apparatus, comprising: an assembly surface comprising an array of electrodes arranged such that each of a plurality of chiplets can be positioned to cover at least one of the electrodes, each of the electrodes capable of being individually activated to apply a clamping force field to one of the chiplets; a field generator that applies a rotation field that is orthogonal to the clamping force field applied by the electrodes, the field generator comprising an input that selects an angle of the rotation field; and a processor coupled to the electrodes and the field generator, the processor operable to: determine a desired orientation angle of a first subset of the chiplets; activate one or more of the electrodes so that a second subset of the chiplets different than the first subset is kept from rotating by the clamping force field applied by the one or more of the electrodes; and while the clamping force field is being applied, apply the rotation field at a selected angle to cause the first subset of the chiplets to be oriented at the desired orientation angle.
 2. The apparatus of claim 1, wherein the rotation field comprises a magnetic field and wherein the clamping force field comprises an electrostatic field.
 3. The apparatus of claim 2, wherein the magnetic field is generated by a first pair of coils located on a first pair of opposite sides of the assembly surface and a second pair of coils located on a second pair of opposite sides of the assembly surface different from the first pair of opposite sides, the angle of the rotation field selectable by varying first and second currents applied to the respective first and second pairs of coils.
 4. The apparatus of claim 2, wherein the magnetic field is generated by a pair of magnets, the angle of the rotation field being selected via relative rotation between the magnets and the assembly surface.
 5. The apparatus of claim 2, wherein the chiplets comprise patterned magnetic material such that the chiplets will align with the magnetic field.
 6. The apparatus of claim 2, wherein each of the second subset of chiplets are over two or more of the electrodes, wherein activating the electrodes comprises selecting, for each of the second subset of chiplets, one of the two or more electrodes that will minimize an induced electric field that interferes with the magnetic field.
 7. The apparatus of claim 1, wherein applying the rotation field comprises time-varying a direction of the rotation field around the selected angle to induce small movements in the first subset of the chiplets, the small movements overcoming stiction between the first subset of the chiplets and the assembly surface.
 8. The apparatus of claim 1, wherein the chiplets comprise 2-D materials.
 9. An apparatus, comprising: an assembly surface comprising an array of electrodes arranged such that each of a plurality of chiplets can be positioned to cover at least one of the electrodes, each of the electrodes capable of being individually activated to apply a clamping force field to one of the chiplets; at least one magnet that is configurable via an input to exert a magnetic field on the chiplets at a selectable angle relative to the assembly surface; and a processor coupled to the electrodes and the at least one magnet, the processor operable to: determine a desired orientation angle of a first subset of the chiplets; activate one or more of the electrodes so that a second subset of the chiplets different than the first subset is kept from rotating by the clamping force field applied by the one or more of the electrodes; and while the clamping force field is being applied, activate the at least one magnet at a selected angle to cause the first subset of the chiplets to be oriented at the desired orientation angle.
 10. The apparatus of claim 9, wherein the at least one magnet comprises: a first pair of coils located on a first pair of opposite sides of the assembly surface; and a second pair of coils located on a second pair of opposite sides of the assembly surface different from the first pair of opposite sides, the selectable angle of the magnetic field selectable by varying first and second currents applied to the respective first and second pairs of coils.
 11. The apparatus of claim 9, wherein the at least one magnet comprises a pair of magnets on opposite sides of the assembly surface, the apparatus further comprising a motor that induces relative rotation between the pair of magnets and the assembly surface, the selectable angle of the magnetic field selected via the motor.
 12. The apparatus of claim 9, wherein the chiplets comprise patterned magnetic material such that the chiplets will align with the magnetic field.
 13. The apparatus of claim 9, wherein each of the second subset of chiplets are over two or more of the electrodes, wherein activating the electrodes comprising selecting, for each of the second subset of chiplets, one of the two or more electrodes that will minimize an induced electric field that interferes with the magnetic field.
 14. The apparatus of claim 9, wherein activating the at least one magnet at the selected angle comprises time-varying a direction of the magnetic field around the selected angle to induce small movements in the first subset of the chiplets, the small movements overcoming stiction between the first subset of the chiplets and the assembly surface.
 15. A method comprising: arranging a plurality of chiplets on an assembly surface such that the chiplets do not interfere with one another, the assembly surface comprising an array of electrodes and the chiplets arranged such that each chiplet is over at least one of the electrodes; determining a desired orientation angle of a first subset of the chiplets; activating one or more of the electrodes so that a second subset of the chiplets different than the first subset is kept from rotating by a clamping force field applied by the one or more electrodes; and applying a rotation field that is orthogonal to the clamping force field applied by the one or more electrodes, an angle of the rotation field selectable to cause the first subset of the chiplets to be oriented at the desired orientation angle.
 16. The method of claim 15, wherein the rotation field comprises a magnetic field and wherein the clamping force field comprises an electrostatic field.
 17. The method of claim 16, wherein the magnetic field is generated by a first pair of coils located on a first pair of opposite sides of the assembly surface and a second pair of coils located on a second pair of opposite sides of the assembly surface different from the first pair of opposite sides, the angle of the rotation field selectable by varying first and second currents applied to the respective first and second pairs of coils.
 18. The method of claim 16, wherein the magnetic field is generated by a pair of magnets, the angle of the rotation field selectable via relative rotation between the magnets and the assembly surface.
 19. The method of claim 16, wherein each of the second subset of chiplets are over two or more of the electrodes, wherein activating the electrodes comprising selecting, for each of the second subset of chiplets, one of the two or more electrodes that will minimize an induced electric field that interferes with the magnetic field.
 20. The method of claim 15, wherein applying the rotation field comprises time-varying a direction of the rotation field around the angle to induce small movements in the first subset of the chiplets, the small movements overcoming stiction between the first subset of the chiplets and the assembly surface.
 21. The method of claim 15, further comprising activating an additional one or more electrodes under at least one chiplet of the first subset to fix a rotation point of the at least one chiplet while allowing the at least one chiplet to rotate around the rotation point in response to the rotation field. 